In many circumstances, reliable and stable removal of heat from a source of heat to the surrounding environment, as well as delivery of heat to a working chamber of a processing apparatus may be a critical issue, wherein violation of normal thermal conditions may lead to abnormal operation of the apparatus or even to a more serious consequences. One such example is removal of heat from RF coils used in inductively coupled plasma reactor for excitation of plasma simultaneously with maintaining the working gas inside the plasma-generation chamber at a predetermined operation temperature. A typical inductively-coupled plasma (hereinafter referred to as ICP) reactor has a housing made, e.g., from a dielectric material, with a flat top surface which supports a flat spiral inductive coil, e.g., an RF coil designed for excitation of plasma in the reactor. The RF coil is separated from the plasma by quartz or other dielectric plate. The RF coil may also be arranged on the outer side surfaces of the reactor. The aforementioned coils are activated by an alternative a.c. voltage from a high-power generator operating, e.g., on a frequency of 2 or 13.72 MHz. The interior of the reactor is maintained under a deep vacuum from fractions mTorr to 10–20 mTorrs. A working gas required for generation of specific plasma is supplied to the reactor and should be maintained during generation of plasma at a predetermined stable temperature. Examples of such gases are oxygen, chlorine, NF3, CF4, etc., or mixtures of the above.
An example of an ICP reactor with a flat spiral RF coil placed onto the top of the plasma reactor is disclosed, e.g., in U.S. Pat. No. 6,422,172 issued in 2002 to J. Tanaka, et al. There exist many other ICP reactors of the aforementioned type and they become so common that some of them are even included into handbooks (see “Industrial Plasma Engineering” by J. Reece Roth, Vol. 1, Institute of Physics Publishing, 1995, pp. 412–413.)
In operation of the ICP reactor, the RF coils consume a significant power which may reach 10 kW or higher. A significant part of this energy is converted into Joule heat (e.g., up to 1 kW or higher). In other words, the RF coils of an ICP reactor work under very severe thermal-exchange conditions required for removal of the excessive heat. Therefore, in a majority of real ICP reactors, the RF coil assemblies are made in the form of hollow spiral tubes having a round or rectangular cross-section for circulation of a cooling agent through the coil. Such RF coils are normally made from a material of high electrical and thermal conductivity, e.g., from copper or the like, and the cooling medium comprises deionized water.
However, it is understood that during operation of the plasma reactor, heat is generated not only in the coils, but also in the volume of the generated plasma inside the reactor. In some cases, the technological process requires that this heat must be removed also from the plasma-generation chamber, e.g., for maintaining the working gas of the plasma-generation chamber at a relatively low temperature. In this case, the coolant circulating through the interior of hollow RF coils is used as a medium for removal of heat not only from the RF coils but from the plasma-generation chamber as well.
On the other hand, operation conditions may exist when gas in the same reactor should be maintained at a relatively high temperature and when the heat generated by plasma itself appears to be insufficient and the use of an external heat source is required.
Designs of such reactors should satisfy a number of conditions, some of which are contradictory with each other. For example, they have to operate efficiently under conditions of a wide temperature range from room temperature to 200° C. or sometimes to 300° C. Heat transfer interface between a heat source, e.g., RF coils, and the interior of the processing chamber should provide efficient transfer of heat and at the same time should function as an efficient electric insulator between turns of the coil. Heretofore, in a majority of cases the above condition of electrical non-conductivity was met by merely producing the coil-supporting cover of the reactor from a dielectric material, while the condition of good thermal conductivity was met by providing heat-transfer contact in interface between the coil and the reactor cover. In accordance with the existing practice, the thermal interface was carried out either through rigid contact or by utilizing thermally conductive elastic insulators such as, e.g., T-gon 200 Series insulators produced by Thermagon, Inc., OH, USA. However, the best thermally conductive elastic insulators of the aforementioned type are rated maximum to 200° C. and therefore are not applicable for high-temperature applications and in reality are effective at working temperature that do not exceed 150–180° C.
Furthermore, in some applications, e.g., RF plasma reactors, the materials that participate in heat-transfer and electrically insulated interface, such as the RF coil, which is made from a material of high electrical conductivity, e.g., as copper, and the dielectric material of the reactor cover, such as ceramic or quartz, have different coefficients of thermal expansion. This difference may be as high as 10 times or more. At the same time, the diameter of the spiral coil assembly and the diameter of the reactor cover may reach 500 mm or greater. It is obvious that when the working temperature changes in a very wide range, e.g., from room temperature to 300° C. or more, the relative displacements between RF coil and the reactor cover become significant, so that peripheral areas of the coil will be shifted in the radial inward and outward directions with respect to the surface of the cover for a distance of several millimeters or more. It is understood that such shifts of the coil will violate the plasma distribution pattern on the treated object, such as a semiconductor substrate. Another consequence of such differences in coefficients of thermal expansion is thermal warping of the contacting parts, i.e., the spiral coil and the reactor cover, which results in violation of uniformity of heat-transfer contact between the two thermally-conductive links. In other words, the aforementioned condition leads to violation of thermal interface and hence to nonuniformity of the generated plasma. More specifically, violation of spatial uniformity in heat transfer conditions may leads to local non-uniformities of treatment, and in the worst case to overheating and even to damage of the apparatus components.
It should be noted that in apparatuses such as RF plasma reactors that utilize vacuum in working chambers the vacuum generate an additional force acting in the direction of violation of the interface. The above condition is aggravated with the raise of the working temperature whereby the sagging of the vacuum chamber walls is increased.
On the other hand, when the temperature of the working gas in reactors or apparatus based on high-temperature processes is insufficient, these apparatus cannot effectively operate.
The applicants are not aware of any existing heat-transfer devices that maintain stable heat-transfer contact between parts subject to significant thermal deformations and operating under temperatures up to 350° C.